Technical Field
The present disclosure relates generally to audio signal processing, and more specifically to adjusting the amplitude of a digital audio signal in the frequency domain to control the perceived loudness of the audio signal at a desired level.
Description of the Related Art
In the literature, many attempts have been made to tackle the issue of varying volume across different materials, e.g., television programs. Common techniques include automatic gain control (AGC) and dynamic range control (DRC). In short, both techniques measure the level of an audio signal in some manner and then gain-modify the signal by an amount that is a function of the measured level. Some techniques also use scene analysis techniques to help calculate the gain applied to the input signal. These techniques can be implemented in time domain or frequency domain. A previously designed a single-band loudness leveller in the time domain is disclosed in U.S. Pat. No. 8,437,480 B2.
The conventional methods used in frequency domain techniques normally include: 1) split samples corresponding to a digitized audio signal to overlapping blocks that are then transformed to frequency domain using STFT or other transform; 2) separate the frequency spectrum to several sub-bands and objectively determine the perceived loudness of the audio signal; 3) scale the loudness of individual sub-bands, adjusting the overall perceived loudness while preserving the perceived loudness spectrum or timbre and 4) transform the data back to time domain.